You're right and they're right: my 1974 MOI28 is hull number 107, so 61 for yours should be about correct. But those numbers are just the count, every manufacturer could have a 61 or 107. There IS a lengthier number (12 digits sounds about right) that is unique across all hulls by whatever manufacturer.
All boats manufactured or imported on or after November 1, 1972 must bear a HIN. The HIN is a 12 character serial number that uniquely identifies your boat. The HIN has an important safety purpose. It enables manufacturers to clearly identify for boat owners the boats that are involved in a defect notification and recall campaign. A HIN is not the same as a State registration number, which may be required to be displayed on the bow of your boat. The HIN is a Federal requirement; your boat’s registration number is a State requirement similar to the license plate on your car. The HIN, however, is required to be shown on the State certificate of registration.
The boat manufacturer must display two identical hull identification numbers, no less than one-fourth of an inch high, on each boat hull. The primary HIN must be permanently affixed (so that it can be seen from outside the boat) to the starboard side of the transom within two inches of the top of the transom, gunwale, or hull/deck joint, whichever is lowest.
On my hull, the number is embossed into the gelcoat just under the rubrail on the starboard side of the transom. If I squint and look at it just right as the sun goes down, I can see it. Barely.
I can feel it, though, fairly easily. And I did a pencil rubbing of it once to be sure I had the numbers right. So that may help if yours has eroded as much as mine has.
S/V las Brisas